An image display device using a hold type display device, e.g., a liquid crystal display device, has such a problem that quality of a moving image deteriorates (edge blurs).
The deterioration of quality (blurred edge) of a moving image which occurs in a conventional hold type display device will be explained by taking, as an example, a case where a region whose image signal luminance level is 75% moves horizontally on a background whose image signal luminance level is 25% as illustrated in FIG. 2.
FIG. 16 illustrates a distribution of input image signal luminance levels respectively corresponding to the pixels in a single horizontal line of a single screen corresponding to image signals inputted to a certain frame at the time of the aforementioned image display.
FIG. 17 illustrates temporal transition of the display luminance distribution in the conventional hold type display device. This temporal transition is observed when such an image moves horizontally. Generally, an observer viewing an image watches a horizontally moving object, so that the observer recognizes, as a luminance level which can be felt by eyes, an integration quantity obtained by integrating display luminance levels in directions indicated by arrows. In FIG. 18, for easier illustration of input image signal luminance levels, a luminance response time of the display device is ignored, and numeric values respectively indicate states of luminance levels of the pixels in the single horizontal line of the single frame in case where the single frame period is divided into eight periods.
Distribution felt by the observer's eyes is an integration quantity of the luminance levels. In case where a velocity at which an object (region whose luminance level is 75%) moves is 8 pixels/frame, the integration quantity of the luminance levels is an average level of a sum of the luminance levels in the time periods added up in the arrow directions. FIG. 19 is a graph indicative of the distribution of the luminance levels. This luminance level distribution is such that there are lines each indicative of a region whose luminance level is 25% and there is a line indicative of a region whose luminance level is 75% and these lines are connected by line segments each of which is inclined in the vicinity of a border between the regions. A horizontal width of this inclined line segment is recognized as the blurred edge. This causes the deterioration of quality of a moving image in the hold type display device.
The most simple method for reducing the blurred edge is such that a minimum luminance level (black) display period is provided in a part of a display single frame. However, according to such a method, bright and dark states are repeated in the entire screen at each cycle of a single frame, which results in occurrence of flickers. Further, also in case where an input image signal is maximum, a minimum luminance level display period is necessarily provided in a single frame period, which results in drop of the luminance level.
The following describes a case where a width of the region whose luminance level is 75% is smaller than a movement amount on the background whose luminance level is 25% in a single frame period as illustrated in FIG. 8.
FIG. 20 illustrates a distribution of input image signal luminance levels corresponding to pixels on a single horizontal line of a single screen corresponding to image signals inputted to a certain frame at the time of the image display illustrated in FIG. 8. FIG. 21 illustrates temporal transition of a display luminance distribution in a conventional hold type display device. This temporal transition is observed when such an image moves horizontally. FIG. 22 is a table showing numeric values respectively indicating states of luminance levels of the pixels in a single horizontal line of a single frame period in case where the single frame period is divided into eight periods.
In case where a velocity at which an object (region whose luminance level is 75%) moves is 8 pixels/frame, an integration quantity of the luminance levels is an average of the luminance levels in the time periods added up in the arrow directions, and is the distribution felt by the observer's eyes is the integration quantity. FIG. 23 is a graph indicative of the distribution of the luminance levels.
As illustrated in FIG. 23, the large blurred edge as exemplified in FIG. 19 does not occur, but a luminance level of an object which essentially moves with a luminance level of 75% significantly drops to 44%. This means that a moving object appears to be darker than it is. Also this point causes the deterioration of quality of a moving image.
Contrary to the aforementioned example, the background has a high luminance level and a moving region has a low luminance level, so that the luminance of the moving region appears to be higher. This phenomenon causes the deterioration of quality of a moving image.
Patent Literature 1 proposes a method for reducing the blurred edge without generating any flickers. This method is such that: as illustrated in FIG. 24, a virtual frame image corresponding to a temporal midpoint between two consecutive frames is estimated and generated, and the thus generated virtual frame image is inserted between the two consecutive frames so as to reduce the blurred edge, thereby preventing the deterioration of quality of a moving image.
However, in the method such as Patent Literature 1, it is difficult to exactly estimate an image signal between the two frames, and estimation failure may cause an error.
The following focuses on a single horizontal line of a screen in case where an object whose luminance level is 75% moves horizontally on the background whose luminance level 25% as illustrated in FIG. 2. For example, a luminance level distribution of input image signals in an (N−1)th frame is as illustrated in FIG. 25(a), and a luminance level distribution of input image signals in an N th frame is as illustrated in FIG. 25(b). At this time, in case where it is possible to exactly estimate and generate a virtual frame corresponding to a temporal midpoint between the (N−1)th frame and the N th frame, the luminance level distribution is such that the region whose luminance level is 75% is positioned at a midpoint between the (N−1)th frame and the N th frame as illustrated in FIG. 25(c). However, it is difficult to completely exactly estimate an image signal between the two frames, and estimation failure may cause an error. If the virtual frame corresponding to the temporal midpoint contains an error, this results in a state illustrated in FIG. 25(d). As illustrated by an arrow, a pixel at a position where a luminance level should be 75% has a luminance level of 25%.
In case where an error occurs in the virtual frame corresponding to the temporal midpoint, numeric values respectively indicating states of luminance levels in a single frame period are as illustrated in FIG. 26, and a distribution of luminance level integration quantities viewed by an observer watching a moving object is as illustrated in FIG. 27. This example shows that: in the vicinity of a right edge of the region whose luminance level is 75%, estimation failure does not occur concerning the virtual frame, so that there is no problem in the distribution of the luminance level integration quantities, and a width of the blurred edge is improved due to the original effect compared with the case of the conventional hold type display device illustrated in FIG. 19. However, due to an influence exerted by occurrence of the estimation failure concerning the virtual frame, there are level differences in a distribution waveform of the luminance level integration quantities. The level differences cause the deterioration of quality, e.g., an image noise, and the like.
On the other hand, Patent Literature 2 discloses a technique in which an image free from a high spatial frequency and an image emphasizing the high spatial frequency are repeatedly displayed so as to prevent a moving image from blurring. However, according to the technique of Patent Literature 2, frame images are generated from a single input image, so that there occurs deviation in a relationship between time space and luminance centroid with respect to an observer who watches an object moving on a screen, which raises such a problem that it is impossible to appropriately keep a distribution of integrated luminances which are observed at a front end and at a rear end of a displayed object. Further, positive values of a high spatial frequency component are removed, which results in such a problem that a luminance of an emphasized frame is always high and flickers occur in the entire screen.
A technique disclosed by the following Patent Literature 3 can solve the problems in Patent Literature 1 and Patent Literature 2.
Patent Literature 3, which regards an invention also invented by the present inventors, is such that: a single frame period is divided into a sub frame period A and a sub frame period B, and a blurring process is carried out in the sub frame period A, and an emphasizing process is carried out in the sub frame period B, so as to improve quality of a moving image, thereby solving the problem in Patent Literature 1.
Further, Patent Literature 3 discloses a technique in which: a blurred frame image is generated, and an average value between the latest frame and a previous frame is used. Thus, according to Patent Literature 3, it is possible to realize an appropriate relationship between space time and luminance centroid with respect to the observer who watches an object moving on the screen, so that it is possible to keep an appropriate distribution of integrated luminances observed at a front edge and at a rear edge of a displayed object. Further, according to Patent Literature 3, a broad luminance of the blurred frame and a broad luminance of the emphasizing frame are identical to each other, so that no flickers occur in the entire screen.
However, the technique disclosed by Patent Literature 3 has the following problem.
That is, in carrying out the emphasizing process, in case where a level of an input image signal which is originally inputted from the outside and which has not been subjected to the emphasizing process is approximate to a minimum image signal level which can be displayed or approximate to a maximum image signal level which can be displayed, further emphasis cannot be carried out, so that it may be impossible to sufficiently carry out the emphasizing process. The insufficient emphasizing process makes it impossible to improve quality of a moving image. This is a problem in this technique.
Citation List
Patent Literature 1
Japanese Patent No. 3295437 (issued on Jun. 24, 2002)
Patent Literature 2
US2006/0227249 (Publication Date: Oct. 12, 2006)
Patent Literature 3
WO2007/052441 (Publication Date: May 10, 2007: International publication)
Patent Literature 4
Japanese Patent Application Publication, Tokukai, No. 2002-351382 A (Publication Date: Dec. 6, 2002)